It is known to use energy of ultrasonic frequencies for various purposes. It is particularly known to apply ultrasound power to liquids for commercial and industrial processes such as cleaning, soldering, emulsification, pickling of steel and pasteurization.
In these and other applications, it is further known to use forms of frequency and amplitude modulation of the applied power drive frequency in order to provide performance which is improved over continuous wave (CW) operation.
Thus, the prior art teaches application of acoustic energy at swept frequencies in the range of 0.5 kHz to 60 kHz for removal of bubbles from a bath of molten glass, for example. Such an operation is known to apply acoustic energy at frequencies resonant to bubbles of different diameters in order to drive the bubbles to pressure wells and thereafter by oscillating smaller bubbles to stir liquid thereabout to facilitate breakup and absorption of the bubbles by the liquid.
Thus, in U. S. Pat. No. 4,398,925 there is disclosed a resonant generator delivering an output signal which is swept through the resonant frequencies of the bubbles. A levitation generator 20 generates a 60 kHz signal which is modulated in a mixer 24 by the output of the resonant generator, whose frequency is controlled by a frequency controller. The modulated signal is provided to an acoustical transducer and is applied thereby to the bath.
In U. S. Pat. No. 3,371,233 a multifrequency ultrasonic cleaning apparatus is disclosed. However, rather than controlling a frequency generator to produce the various frequencies, shock excitation impulses are provided in a random fashion to a rectangular transducer which, because of the various different dimensions thereof, resonates at each of its frequencies, as well as the harmonics thereof, in order to generate simultaneously a wide band of ultrasonic cleaning frequencies. The disclosure thus seeks to avoid exercising of control on the waveforms applied to the transducer, such as tuning the waveforms to desired frequencies. Instead, impulse or square wave excitation is applied to the transducer which itself provides the frequency governing elements. Thus, only a simple pulse generator is used, with fixed, invariable, parameters and thus with no control over operation of the cleaning equipment.
There is accordingly a need in the prior art for controllable pulse generators capable of generating pulses having characteristics which may be controlled to meet certain predefined operational criteria.
In most modern power ultrasonic generators intended for liquid applications, amplitude modulation (AM) of the power drive frequency is also used. Particularly, the AM is a full wave sinusoidal pattern at two times the power line frequency. More complex frequency modulation (FM) techniques are known. Two components may make up the techniques as follows. An auto-tuning system is used to maintain resonance with the output reactive load impedance and a sweep frequency component, periodic with the AM frequency, may be provided. Even in such complex systems, however, resultant advantages are typically a matter of coincidence rather than of optimum design.
It is noted, for example, that in systems of the type described above the AM pattern typically results from full wave rectification of the power line voltage. Thus, an ultrasonic generator functioning as described will have a 120 Hz AM pattern when operated in the United States, where 60 Hz AC is common. However, the same ultrasonic generator, when operated in Europe, will have a 100 Hz AM pattern because 50 Hz AC is the common line voltage. At any rate, neither of the operating frequency envelopes is related to process optimization, but rather is a matter of convenience and easily available waveform.
As to the auto-tuning component of FM, the same is a naturally occurring effect of the feedback system in power oscillators, while a sweep frequency FM is a naturally occurring phenomenon, due to the varying levels of partial saturation of the output inductance as current levels change because of the AM pattern.
In U. S. Pat. No. 3,638,087 a gated power supply is disclosed for reducing power consumption by sonic cleaners. Therein is described a variation of pulse width and repetition rate to produce a pulse-modulated power output. A sonic signal, at fixed frequency, is thus gated and output for pulsed time periods which vary in duration and repetition rate.
The disclosed circuitry includes a gate operated by a pulse generator. The gate passes a sonic control frequency generated by a sonic generator to a power output stage only when the associated pulse generator is at a given voltage level. The length of time that the gate permits an output signal to operate the power output stage is proportional to the pulse width provided thereto. Thus, by varying the width of the voltage from the pulse generator, the length of time that the gate permits a sonic output signal to operate the power output stage is varied. The output from the power stage is connected to a sonic transducer which vibrates a cleaning tank. By varying the pulse width of the signal received from the pulse generator, varying amounts of pulse width modulation or duty cycle can be obtained for the sonic frequency power output. Further, by varying the frequency of the pulse generator, the repetition rate of the pulse width modulation is varied.
In the reference it is disclosed that by varying both the pulse width and pulse frequency, the most efficient type of pulse width modulation for a sonic generator, and thus the most efficient mode of operation for the sonic cleaner, can be experimentally determined with the aid of a Fluke meter. Increased efficiency of operation is illustrated by adjusting the pulse generator to reduce by 90% the time fraction during which the sonic power output is available, to attain significant reduction in cleaning costs, at the cost of a 50% increase in time necessary to perform the cleaning operation.
However, although the reference teaches a modification of duration and repetition rate for bursts in order to attain a single objective by optimization of power usage efficiency, the prior art fails to provide sufficient control over the waveforms of sonic frequency pulses to optimize any application of ultrasonic power to a liquid. The prior art is thus deficient in optimizing application of ultrasonic power to a liquid under any defined criterion or set of criteria.
Generally, it is noted that many low volume power ultrasound applications are known. As a result, many different multiple-criterion sets are applicable for judging the efficacy of the various schemes when used in the different applications. It is thus necessary to optimize the various AM, FM or PM (pulse modulation) schemes according to a number of different criteria for each application. Techniques for variation of one or two characteristics in order to optimize the particular AM, FM or PM waveform to meet the requirements of a single criterion are thus of limited use.
Accordingly, although the above described art discloses some forms of control over applied sonic frequency and amplitude of a driving pulse, there is a need in the prior art for a method and apparatus which provides control over a sufficiently large number of parameters of the applied sonic waveform as to optimize the waveform for any given set of criteria associated with a particular application.